The present invention relates to a video display apparatus and in particular to a video display apparatus that uses a display data channel (DDC) system to communicate device specific information to an external controller.
The American Video Electronics Standard Association (VESA) proposed the DDC Standard. Under the DDC standard a computer system having a computer connected with peripheral equipment such as a monitor, a keyboard, a mouse, and so on may exchange device specific data among the computer and the peripheral equipment through bus lines in accordance with a predetermined protocol.
The DDC system defines several levels. DDC1 mode accomodates unidirectional communication. DDC2 mode allows bidirectional communication. The DDC2 mode is further classified into two levels. DDC2B mode only allows reading of DDC data from peripheral equipment by the computer. DDC2AB mode allows reading and writing DDC data between the computer and the peripheral equipment. The DDC2AB system is also referred to as an ACCESS bus.
Device specific information, such as the device model or serial number, as well as operational information, such as the settings for the vertical and horizontal screen sizes, are stored as DDC data. DDC data is typically stored in a region of non-volatile memory within the display device. DDC data can be accessed by an external controller.
FIG. 1 shows a possible arrangment of a video display device 6 that utilizes a DDC system. DDC data 21 is stored in non-volitile memory 2. DDC data 21 is accesible by a microcomputer 1 via the DDC buses 14a, 14b. An external controller 5 accesses DDC information from the microcomputer 1 through a communications bus 15. A second DDC external controller 4 can access the DDC non-volatile memory 2 via the DDC buses 14a, 14b.
In a system where the DDC data 21 resides only in the DDC non-volatile memory, several difficulties may be encountered that prevent rapid access of DDC data 21 by an external controller 4. For example, since the DDC bus is connected not only to the video display apparatus 6 and the external controller 4 but may also be connected to a mouse or the like, when communication between the mouse and the external controller 4 occupies the DDC bus, the microcomputer 1 cannot gain access to the bus 14. Therefore, communication cannot be carried out between the microcomputer 1 and the DDC non-volatile memory 2.
Additionally, if the microcomputer 1 reads out the DDC data 21 from the non-volatile memory 2 after receiving a command from the external controller 4 requesting the microcomputer 1 to send the DDC data, there will be a delay before the microcomputer 1 can transmit DDC data to the external controller 4. An error will occur if this delay exceeds the time allowed for a reply in the DDC2AB mode.
Also, if the portion of the DDC non-volatile memory 2 holding DDC data is damaged, then the DDC data must be rewritten to a new portion of DDC non-volatile memory 2'. The data must be written in the non-volatile memory 2' from the DDC external controller 4 through the DDC buses 14a, 14b. If the DDC data includes data specific to the video display apparatus, for example, its serial number, then the DDC external controller 4 must be given this information before it can rewrite DDC data to the undamaged DDC non-volatile memory 2'.
Since the DDC non-volatile memory 2 is connected to a bus that is accessible outside the video display apparatus 6, the DDC data 21 stored in the non-volatile memory 2 may be corrupted.